Origins of Lithium/Sodium Reverse Permeability Selectivity in 12-Crown-4-Functionalized Polymer Membranes

ES Zofchak and ZD Zhang and BK Wheatle and R Sujanani and SJ Warnock and TJ Dilenschneider and KG Hanson and S Zhao and S Mukherjee and MM Abu- Omar and CM Bates and BD Freeman and V Ganesan, ACS MACRO LETTERS, 10, 1167-1173 (2021).

DOI: 10.1021/acsmacrolett.1c00243

Direct lithium extraction via membrane separations has been fundamentally limited by lack of monovalent ion selectivity exhibited by conventional polymeric membranes, particularly between sodium and lithium ions. Recently, a 12-Crown-4-functionalized polynorbornene membrane was shown to have the largest lithium/sodium permeability selectivity observed in a fully aqueous system to date. Using atomistic molecular dynamics simulations, we reveal that this selectivity is due to strong interactions between sodium ions and 12-Crown-4 moieties, which reduce sodium ion diffusivity while leaving lithium ion mobility relatively unaffected. Moreover, the ion diffusivities in the membrane, when scaled by their respective solution diffusivities and free ion fractions, can be collapsed to an almost universal relationship depending on solvent volume fraction.

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